The ATLAS (Atmospheric Laboratory for Application and Science) missions were part of NASA's ESE (Earth Science Enterprise) program, formerly called MTPE (Mission to Planet Earth). The ATLAS payloads, mounted on two Spacelab pallets in the cargo bay of the Space Shuttle, investigated specifically how Earth's atmosphere and climate are affected by the sun and by the products of industrial complexes and agricultural activities.

Experiments flown on the three ATLAS missions gathered data throughout the sun's 11-year activity cycle. There is a “core” of instruments which was the same on all ATLAS missions, and additional instruments unique to several of the Shuttle flights. An important goal of the ATLAS program is to provide measurements that relate to and coincide with other instruments that are flying on other satellites. There is a coordinated underflight program for correlative measurements such as: UARS, NOAA-POES, ERBS, EURECA, TOMS (on Nimbus-7 and Meteor-3-6). 1)2)3)4)

The ATLAS sensor complement was mounted ontoSpacelab pallet(s), providing the instruments with all service functions such as power, cooling, commanding, and communications links. The open, U-shaped “pallet” platform was a component of the reusable Spacelab equipment, provided by ESA (European Space Agency) in 1981 as its contribution to the Space Shuttle program. The payload service devices like power supply, command and data handling system, and temperature control system, were housed in a pressurized container called the igloo (also standard Spacelab equipment) located in front of the pallet.

ATLAS-1 consisted of 12 international experiments and supporting hardware mounted on a two-Spacelab-pallet train plus igloo in the Orbiter payload bay, and one experiment contained in two canisters mounted on one adapter beam assembly (SSBUV, see Figure 1). Overall objective: a) study of the chemical makeup of the atmosphere between approximately 15 and 100 km above the Earth's surface, b) measurement of the total energy contained in sunlight and how that energy varies, c) investigation of how the Earth's electric and magnetic fields and atmosphere influence one another, and d) examine sources of ultraviolet light in the universe. Many of the experiments are also scheduled for later ATLAS missions, so the data gathered during ATLAS-1 was the first in a series of long-range studies that measured changes in the atmosphere and the sun. 5)

ATLAS-1 had periods of solar pointing, Earth-limb pointing, and additional special attitudes throughout the mission. The orbiter is the primary experiment pointing system; however, some of the experiments have limited fine-pointing capability for enhanced pointing accuracy.

Correlative measurements to UARS. ATLAS-2 payloads gathered data on the relationship between the sun's energy output and the Earth's middle-atmosphere chemical makeup. Study of how these factors affect the Earth's ozone level.

Figure 2: View of the ATLAS payload in the cargo bay of the Space Shuttle (image credit: NASA)

Figure 3: Overview of instruments of the ATLAS-1 mission positioned on pallets in the cargo bay (image credit: NASA)

Sensor complement (ATLAS core instruments):

ATMOS (Atmospheric Trace Molecule Spectroscopy):

ATMOS is a NASA/JPL instrument and experiment within the ESE program, PI: Michael R. Gunson. This is an infrared absorption instrument which works during occultations (sunrise, sunset) and measures a wide variety of species with good vertical resolution. The objective of ATMOS is to make global measurements of the composition of the troposphere, stratosphere, and mesosphere. The spectrometer simultaneously measures the concentrations of gases that are involved in the complex chemical and radiative interactions occurring at altitudes between 10-150 km. With the instrument's ability to detect these trace gases in concentrations of lower than 1 part per billion, its data are used to make critical tests of theoretical models that describe the physics and chemistry of the stratosphere. 8)9)10)

The ATMOS instrument was built at the Honeywell ElectroOptics Center. The instrument consists of the following elements: the optical sensor and the electronic assemblies. All the elements of the optical sensor are mounted to an aluminum baseplate that in turn is mounted via vibration isolators to a substructure assembly. ATMOS employs a high-resolution FT (Fourier Transform) Michelson interferometer and limb viewing in the spectral region of 2.2-16 µm to measure the absorption of sunlight by the atmospheric molecules.

The ATMOS experiment was already very successful on Spacelab-3 (flown April 29-May 5, 1985). During ATLAS-3, the ATMOS instrument also had a video camera to record pictures of the sun during sunrises and sunsets (pointing confirmation).

PI: G. Hartmann, MPAe, Lindau, Germany. Measurement of millimeter-wave emission from the atmosphere. As a limb emission instrument, it gets nearly global coverage. Determination of the following parameters:

The MAS experiment is a joint project of: University of Bremen (K. Künzi, scientific investigation), University of Bern (hardware), MPAe Lindau (data processing and scientific investigation) and the Naval Research Lab (Washington). 11)12)13)

The MAS instrument on ATLAS was also used for comparison with similar instruments (MLS) on UARS (Upper Atmosphere Research Satellite). MAS recorded important measurements on ozone and chlorine monoxide, a key trace molecule involved in the destruction of ozone.

SUSIM (Solar Ultraviolet Spectral Irradiance Monitor):

SUSIM is a dual dispersion spectrometer instrument of NRL (Naval Research Laboratory), Washington, D. C., PI: G. Brueckner. SUSIM measures solar UV flux as a function of wavelength from 110 to 410 nm [compatible with SUSIM on UARS (Upper Atmosphere Research Satellite), also for calibration of UARS instrument]. SUSIM can also observe ozone and molecular oxygen profiles by occultation during solar observing orbits. Heritage: SUSIM flew on Spacelab-2 (STS-19, July 29-August 6, 1985) and on UARS. 14)15)16)

SOLCON is of IRMB, Brussels, Belgium, PI: R. Crommelynck. Measurement of the solar constant. The SOLCON instrument is a cooperative effort of IRMB, Space Science Dept. of ESA, and LaRC. Measurement of the absolute value of the total solar irradiance (and long-term variations). The technique used is to compare the heating of a cavity exposed to sunlight with the temperature of another cavity that is not exposed to sunlight. SOLCON consists of two parts: the DPU (Digital Processing Unit) providing the interface to the ATLAS pallet, and the absolute radiometer with its electronics. SOLCON is a differential absolute radiometer with two channels. The incident radiative energy absorbed in a cavity is compared to electrical energy generated by the Joule effect, taking into account the nonequivalence of energies, as well as the geometric, thermal, optic and electrical characteristics of the instrument. 17)18)

When the SOLCON radiometer is pointed toward the sun, one shutter of the cavity is opened. The heat balance system compensates for the added heat until the heat fluxes are again balanced between the open and the closed cavities. The shutter is then closed, and power is adjusted to its original value automatically. The difference in the power required to maintain a heat balance between the two cavities during open and closed operations in a function of total solar radiation.

The SOLCON radiometer on the ATLAS missions is an improved version of SOLCON flown on Spacelab-1. Instrument accuracy to within 0.1% of the solar constant, and about 0.01% precision. SOLCON is a high-resolution, self-calibrating radiometer with a digital processing/converter unit. The mass of the SOLCON experiment is 9.962 kg. Its power consumption is 13.6 W and its date rate is 60 bit/s.

ACRIM-II was developed at JPL, PI: R. C. Willson. ACRIM is a pyrheliometer measuring the total solar irradiance (solar constant) with a technique similar to that of SOLCON. ACRIM-II was initially referred to as ACR (Active Cavity Radiometer) module. 19)

SOLSPC is a radiometer of CNRS, France, PI: G. O. Thuillier. Measures the solar irradiance from 200-2400 nm using three double spectrometers and an on-board calibration device. SOLSPEC can also observe abundances of ozone by measuring the backscatter of specific UV and VIS wavelengths during nadir operations. SOLSPEC heritage: flew on Spacelab-1 (STS-9) in November 1983. The SOLSPEC instrument on ATLAS had a mass of 32 kg and a power consumption of 32 W (one spectrum could be obtained in 11 minutes). 20)21)

The B-USOC (Belgian User Support and Operations Centre), located on on the premises of the Space Pool in Uccle, Brussels, Belgium, covered the support and performance of space missions and facilities since 1992, and include: EURECA (SOVA, ORA and SGF); IML-2 (BDPU); Atlas-2 and Atlas-3 (SOLCON and SOLSPEC); EuroMIR 94, 95/96 (CSK-1); RMS (2-Rip and TITUS); MIR 95 (MIRAS); LMS (BDPU); Neurolab (ALFE); and SpaceHab 98 (BIOBOX/HUDERM, BIOBOX/MARROW,and AGHF-2).

SSBUV was developed at NASA/GSFC, PI: E. Hilsenrath. Uses UV backscatter in nadir to measure vertical profiles of ozone in the stratosphere and in the lower mesosphere from 180 to 450 nm. Note: SSBUV is not a core instrument; it is a separate ATLAS Shuttle payload (co-manifested with ATLAS and integrated into the ATLAS science plan). 22)23)24)

SSBUV is a sensor of SBUV/2 heritage (from Nimbus-7 and NOAA-9 satellites onward). Objective: Calibration of long-term satellite ozone data sets with complementary Shuttle flights. The SSBUV instrument on the ATLAS missions is the SBUV/2 engineering model now flying on NOAA satellites.

The first Shuttle flight with SSBUV instrumentation occurred on Oct. 18-23, 1989 on the Shuttle Atlantis (STS-34). Throughout the Shuttle flight period coincident observations were taken with the SBUV on Nimbus-7 and the SBUV/2 on NOAA-9 and NOAA-11 satellites. The SSBUV spectrometer is located in a GAS canister attached to the side of the Shuttle's cargo bay. A motorized door assembly opens up to allow the SSBUV to view the Earth and the sun.

The SSBUV instrument and its flight support electronics, power, data and command systems are mounted in the Shuttle's payload bay in two flight canisters that, together, have a mass of 410 kg. The Instrument Canister holds the SSBUV instrument, its aspect sensors and in-flight calibration system. Once in orbit, a motorized door assembly opens the canister, allowing the SSBUV to view the Sun and Earth. The canister closes, providing contamination protection, while SSBUV performs in-flight calibrations. The Support Canister contains the avionics, including the power, data and command systems.

Additional ATLAS sensors were:

ALAE (Atmospheric Lyman-Alpha Emissions):

ALAE is an instrument of CNRS, France. Uses on-board hydrogen and deuterium cells to measure thermospheric/exospheric H and D concentrations, as well as Lyman alpha amounts in the interplanetary medium. ALAE was flown on ATLAS-1 (and also on Spacelab-1).

ISO (Imaging Spectrometric Observatory):

ISO is an instrument of NASA/MSFC. The spectrometer measures low-light observations in daylight and on the night-side of the Earth. There are five spectral bands from 30-1300 nm. Some parameters of the ISO are: 26)

- Temporal Resolution: 4 second frame time with two 2 second sub frames.

The five spectrometers are each optimized for a portion of the spectrum by the choice of mirror reflective coatings and detector photocathode materials. The full spectral range for each spectrometer is covered in a total of 11 grating steps. The instrument recorded spectra of the atomic oxygen green line, which had not been measured before in the daytime mesosphere. Also, basic measurements were obtained of the hydroxyl radical in the same region. ISO was flown on Spacelab-1 and ATLAS-1.

AEPI is a NASA instrument built by the Lockheed Palo Alto Research Laboratory. The objectives of this experiment were the study of optical emissions from the upper atmosphere/ionosphere and from the Space Shuttle environment: 1) investigation of ionospheric transport processes by observing positive magnesium (Mg) ions, 2) studies of optical properties of artificially induced electron beams, 3) measurement of electron cross sections for selected atmospheric species, 4) studies of natural airglow, and 5) studies of natural auroras. 27)28)

Images of the atmosphere were produced by a low light-level television camera with special lenses and filters, a photon counting array, and associated electronics. The filters of the instrument were used to detect faint emissions from metastable oxygen, magnesium ions, and other atmospheric elements in the 200 to 750 nm spectral region. A second low-light-level unfiltered (spectral continuum) TV camera consisted of an Augeniux 50 mm f/0.95 lens focused at infinity, whereby an image was formed on a single-stage microchannel plate intensified inverter tube, coupled to an uncooled CCD via a fiberoptics taper. A dedicated experiment processor controlled the detector functions including filter wheel positioning, camera focusing, prism movement, intensifier gain control, and gain control. AEPI was flown on ATLAS-1 as well as on Spacelab-1.

SEPAC is an instrument of SwRI (Southwest Research Institute), San Antonio, TX, USA. SEPAC created and observed several artificial auroras, allowing scientists to observe the structure of the Earth's magnetic field. Together, data from SEPAC and AEPI showed the size and intensity of the artificial auroras and determined the cause of their shape. SEPAC employed an electron beam accelerator and other instruments (AEPI) to carry out active and interactive experiments on and in the Earth's ionosphere. SEPAC was flown on ATLAS-1 - representing a reflight of SEPAC initially flown on the Spacelab-1 mission in 1983. 29)

SEPAC involves the firing of a 1.6 Å 7.5 kV electron beam accelerator from the Shuttle bay to study a variety of phenomena related to vehicle charging and charge neutralization, atmospheric interactions, and virtual-antenna operation. The Plasma Contactor Neutralizer System consists of a 25 cm diameter xenon plasma source, neutral gas source, xenon-storage and control unit, and power supply. The flight plasma source produces 1.5 Å of xenon ion current with an input power of about 210 W and a xenon gas flow of about 2.2 standard liter/h. The neutral gas source is sized to release 1023 atoms of xenon in 100 ms pulses.

The SEPAC instrument was used for controlled experiments that were successful in generating the first artificial auroras ever produced in the Earth's upper atmosphere. By firing a 7.4 kW electron beam into the Earth's upper atmosphere, electrons circling atmospheric nitrogen and oxygen atoms and molecules were excited to higher energy levels. As they resumed to lower levels, they released light, forming high intensity auroras several kilometers in diameter. Forty of the 60 beams produced artificial auroras and were imaged by the Atmospheric Emission Photometric Imaging experiment mounted in Atlantis's payload bay. The energy output of these auroras was greater than the energy input from the beam, indicating that the beam may have triggered larger reactions in the atmosphere. SEPAC was also used to investigate the interaction of ionized and neutral gases in space by injecting over 1,000 xenon gas clouds into the atmosphere. Furthermore, SEPAC generated radio waves with about 100,000 electron beam pulses. The pulses were observed by ATLAS 1 instruments and by over 100 receivers on the ground in the United States and Japan. 30)

FAUST is an instrument of UCB (University of California, Berkeley). Far-ultraviolet images of large-scale phenomena. FAUST was flown on ATLAS-1 and on Spacelab-1 (STS-9).
FAUST is a far ultraviolet (1400-1800 Å) photon-counting imaging telescope featuring a wide field of view (7.6º) and a high sensitivity to extended emission features. During its flight as part of the ATLAS-1 payload aboard the STS-45 mission in March 1992, nineteen deep space nighttime viewing opportunities were utilized by FAUST. 31)32)

FAUST is a compact instrument designed for observations of extended and point sources of astronomical interest. The instrument was developed by the Laboratoire d' Astronomie Spatilale of France and the French Space Agency (CNES). The instrument is an f/1.12 Wynne camera with an effective collecting area of 150 cm2 and a FOV of 7.5º. The imaging capability is better than 2 arcmin in the entire FOV. The detector system uses a microchannel plate image intensifier in conjunction with a 60 exposure, 35 mm film pack of Kodak 103a0. The overall instrument, which includes a sealed container with a mechanical door, is a cylinder 48.8 cm in diameter and 131.3 cm in length.

When used in a photometric mode with a bandpass of approximately 100 nm, the limiting magnitude for a 2 minute observation is V = 17 to 18. Diffuse sources as faint as 27th magnitude/arcsec2 can be detected. In the alternate spectroscopic mode, with a calcium fluoride objective prism which permits a wavelength resolution of between 3 and 20 nm, the limiting magnitude is V = 14 to 15.

The FAUST instrument provided astronomers with their first opportunity to explore wide areas of the sky in the far ultraviolet radiation wavelength range. Most ultraviolet light coming to Earth from space is filtered out by the Earth's atmosphere, making it essential to travel into space to study this radiation firsthand. Previous space-flown ultraviolet instruments have focused on narrow regions of the sky. Before its power failure, FAUST observed the nearby Large Magellanic Cloud galaxy to gain information that may help astronomers better understand the evolution of our own galaxy. A gas trail behind the cloud was observed that could indicate a region of star formation. FAUST also made observations of galaxy clusters in the Virgo, Telescopium, Dorado, and Ophicus constellations.

Grille (Infrared spectrometer):

Grille is a French/Belgian sensor with the objective to measure the chemical makeup of the middle and upper atmosphere 15-150 km altitude range - by observing how chemical species emit or absorb radiation. Grille provides a good resolution in the infrared region (2.5-10 µm) of the spectrum; it measures vertical profiles of: CO, CO2, NO, H2O, CH4, H2O and HCl. The instrument name comes from a special “grille” (literally cricket) used as a window for one leg of its optical system and as a mirror for the other to overcome the limitations of many conventional instruments. The analysis of the Grille measurements lead to the discovery of methane in the mesosphere from 50 km upward, a higher altitude than previously observed or expected. 33)34)35)36)37)

ESCAPE-II (Experiment of the Sun Complementing the ATLAS Payload and for Education-II):

ESCAPE-II is a student-designed and developed experiment located in the cargo bay. ESCAPE-II was built by undergraduate and graduate students of the Colorado Space Grant Consortium at the University of Colorado, Boulder, CO. The objective is to shed some light on how the sun's extreme ultraviolet wavelengths affect the temperature and chemical composition of the upper atmosphere. The release of human-produced chlorofluorocarbons (CFCs) is believed to be largely responsible for the recent seasonal decline in stratospheric ozone levels, most markedly over the Earth's poles. The variability of natural solar radiation is needed to understand the magnitude of human-caused changes in the atmosphere.

CRISTA-SPAS-1 Free-Flyer Platform

CRISTA (Cryogenic Infrared Spectrometer and Telescopes for the Atmosphere) is the name of a major payload on STS-66 (Nov. 3-14, 1994), a mission scientifically combined (co-manifested) with the US ATLAS-3 program. The orbital mean altitude was about 300 km at an inclination of 57º of the ASTRO-SPASfree-flyer platform, released and retrieved in an observational orbit between 40 to 70 km behind the Shuttle. As such, it operates independently, except during communication periods with the orbiter. During these periods, the instrument relays status information via the Shuttle to the ground.

The CRISTA, MAHRSI and SESAM payloads were mounted onto the free-flying ASTRO-SPAS platform (see description of ASTRO-SPAS at end). ASTRO-SPAS was deployed from the Orbiter using RMS (Remote Manipulator System) - and retrieved at the end of the mission using RMS.. The CRISTA/SPAS platform was deployed from the Space Shuttle Atlantis STS-66 on November 4, 1994 for 8 days of free flight.

CRISTA is a limb-scanning spectrometer that measures thermal emissions (4 - 71 µm) of selected trace gases with high spatial resolution in three dimensions. CRISTA is the prime payload of SPAS (PI: D. Offermann, University of Wuppertal, Germany. CRISTA is sponsored by DLR (formerly DARA). It is an infrared instrument with three independent telescopes pointed at the limb (in three directions/dimensions simultaneously). Two telescopes are laterally directed (pointing angles = ±18º from center). Objective: analysis of dynamic processes (winds, wave interaction and turbulence) in the middle atmosphere with the detection of trace gases.

The four spectrometers provide 26 spectral bands, 21 bands from 4.6 - 14.1 µm, and 5 bands from 15.2 - 71 µm. Each telescope has a short-wavelength spectrometer attached; the center telescope measures the longer wavelengths. The spectrometers take one spectrum per second and measure up to 15 trace gases in 26 channels during this time. The telescopes obtain complete altitude observations of these gases within about one minute as the lines-of-sight are scanned through the atmosphere. The high measurement speed and the sensitivity of the instrument are achieved by cryogenic cooling of the CRISTA optics and detectors with liquid helium. Spectral resolution power (λ/Δλ)= 500, spatial resolution: 500 x 650 km in the horizontal direction and 2-3 km in the vertical direction. The CRISTA instrument is contained in a vacuum container cooled. 38)39)

The SPAS platform was three-axis stabilized to point its telescope near the Earth's horizon using mass expulsion thrusters and aiming at a point 62.9 km above the WGS-84 ellipsoid. Attitude determination was achieved by a star tracker-gyro inertial reference unit (IRU), position data was provided by an Alcatel/SEL GPS receiver. The IRU data determined attitude within 0.05º and served as “truth” reference for a comparison with GPS data. 40)

1) The range limit is determined by filter transmission for values marked with a “*”.

2) The range limit is extended by the Gille and House method for all slashed wavelengths (example: 8.6/9.7).

3) Detector types: SiGa (Silicon Gallium), GeGa (Germanium Gallium), BIB (Blocked Impurity Band - BIB was first used with CRISTA-2). In the spectrometer detector column (1), SL, SCS, and SR stand for left, center, and right telescopes and short wavelengths, respectively. SCL is for longer wavelengths in the center telescope only. 41)

GPS-based attitude determinationof SPAS. A TANS Vector (SS/L) receiver, modified by Stanford University software, provided independently attitude and position data. Four GPS patch antennas were mounted on the zenith face of SPAS. The carrier-phase differential GPS (CDGPS) measurements of the TANS Vector receiver served as input for a recursive Kalman filter attitude estimation algorithm.

MAHRSI is an instrument of NRL (Naval Research Laboratory), Washington, D.C., PI: R. R. Conway. Measurement of dayglow in the 190 - 320 nm region with a resolution of 0.02 nm. The prime objective is to measure limb intensity profiles of the resonance fluorescent scattering of sunlight, OH and NO, in the mesosphere and thermosphere (40 - 160 km region in total). From these intensity profiles, vertical density profiles of OH and NO are inferred with a vertical resolution of 2 km. By measuring Rayleigh scattering intensity profiles, the experiment provides precise knowledge of the neutral density and temperature in the mesosphere.

The MAHRSI instrument consists of four subsystems: the telescope/spectrograph assembly, MECA (MAHRSI Electronics Controller Assembly), MDEA (MAHRSI Detector Electronics Assembly), and HVPS (High Voltage Power Supply). The telescope/spectrograph assembly employs an f/7.5 Czerny-Turner spectrograph with a 75 cm focal length behind a 50 cm focal length telescope (57 cm2 aperture) mounted on CRISTA-SPAS. The wavelength range of sensitivity is from 195-320 nm, the spectral resolution is 0.018 nm at 310 nm and 0.026 nm at 215 nm. A spectral bandwidth of about 4 nm is imaged on the focal plane at a given grating position. The grating can be scanned to cover the entire spectral range of the instrument. A highly polished plane scan mirror at the baffled aperture of the telescope controls the vertical motion of the FOV (0.01º x 1.13º). MDEA provides power distribution, operates the intensified CCD (ICCD) detector, and transfers data from the CCD to the microprocessor after each integration. MAHRSI is operated and controlled by MECA (management of the command data interface, control of the dust door, scanning mirror, rotating polarizing filter, and scanned grating). During observation periods, CRISTA-SPAS held MAHRSI's FOV parallel to that of CRISTA's center telescope. 42)

SESAM (Surface Effects Sample Monitor). The objective is to explore the influence of atmospheric atomic oxygen on optical surfaces (used in UV astronomy, tests of “whiteness” samples, development stable of surface layers, etc.). The instrument can accommodate, expose, and test up to 40 samples.

CRISTA-SPAS-2

A reflight of the CRISTA-SPAS-1 mission and payload took place on STS-85 (Aug. 7-19, 1997), labelled as CRISTA-SPAS-2.

The scientific emphasis was on the study of small-scale tracer “filaments” (long, thin regions of differing composition, including temperatures) in the stratosphere. Data analysis of the two CRISTA-SPAS missions may indicate, how these filaments contribute to the transport of ozone. The STS-85 flight offered an opportunity for increased latitudinal coverage of the atmosphere beyond that of the STS-66 mission (data of summer conditions at northern latitudes and the polar night at high southern latitudes). A CRISTA/MAHRSI campaign employing balloons, research aircraft, etc. was conducted in Europe in parallel to the STS-85 mission, in support of CRISTA-SPAS-2.

MAHRSI is of NRL (Naval Research Laboratory), PI: R. Conway. Same instrument as in the STS-66 mission (CRISTA-SPAS-1). During the STS-85 flight, MAHRSI gathered new vertical profile data on the distribution of OH in the mesosphere and upper stratosphere under very different conditions (both seasonal and diurnal) from its previous flight on STS-66. Also, more measurements of nitric oxide were conducted. Researchers hope to gain a better understanding of how hydroxyl (OH) behaves during different seasons around the globe and how it interacts with the behavior of ozone and other trace gases.

IPEX-II (Interferometry Program Experiment 2):

A NASA/JPL experiment, the prime objective was to monitor on-orbit the microdynamic behavior of space-like booms (to serve as input for future interferometry missions). The IPEX-II boom is a 9-bay expandable ADAM mast (2.35 m x 0. 3 m x 0.3 m), developed by AEC-Able, with steel-bracing cables pre-loaded at about 112 kg. The boom was cantilevered to the side of the ASTRO-SPAS platform. The experiment consisted of three phases:

• Investigation of thermal snapping in joint-dominated structures due to temperature gradients. A suite of 24 high sensitivity micro-g accelerometers with collocated temperatures sensors, mounted on the boom, monitored normal operations aboard CRISTA-SPAS in free flight. Data was obtained for five sun-to-shade orbital periods.

• Investigation of structural boom behavior due to induced loads. Two shakers, mounted orthogonally at boom tip, performed random and step-sine tests between 9-300 Hz, with loads from 0.048 to 0.00055 kg to detect any possible nonlinear behavior in the boom.

• Investigation to isolate and to quantify on-board disturbance sources of the CRISTA-SPAS platform and to measure their effect on the boom. This involved gyro and thruster operations and in-between times.

ASTRO-SPAS is the generic name of a reusable platform, designed and built by EADS Astrium GmbH - formerly DASA and prior to that: MBB (Munich, Germany) under DLR contract. The ASTRO-SPAS platform is being used as a self-contained and autonomous free-flyer service structure for special Shuttle payloads with free-flyer requirements for short-duration missions (up to the length of a Shuttle mission). The SPAS structure consists of low-weight, high-stiffness carbon fiber tubes with titanium nodes. Standardized mounting panels are provided for subsystem and payload equipment. The platform is deployed/retrieved by the Shuttle's robot arm RMS (Remote Manipulator System) for a free-flyer mission which may entail separations from the Shuttle up to 100 km. As a service structure, SPAS is particularly suited as a test bed for new science instrumentation and technology demonstrations in space. 43)

The platform overall size is 4.5 m x 1.75 m, its empty mass is about 1240 kg (including service subsystems), it can accommodate a payload up to a total satellite mass of 3600 kg. The platform offers the following service subsystems:

- The inertial pointing mode serves mainly for astronomical observations. The star tracker (CCD camera) measures the position of three guide stars in its FOV of 4.5º x 6º, the gyro package senses rotations.

- The orbit motion mode is used for atmospheric research to point into a specific direction. One axis points into a constant, commandable altitude layer (stabilized to ±2 km). The GPS Tensor instrument and the star tracker provide attitude, position and velocity of the platform.

• TT&C: A scheduled communications link via Shuttle is provided by an S-band transponder with uplink data rates of up to 2 kbit/s and downlink rates of up to 16 kbit/s.

Overview of ASTRO-SPAS free-flyer missions:

• The ASTRO-SPAS free-flyer platform concept was first demonstrated for the MOMS-01 imaging payload on two Shuttle missions: STS 7, June 18-24, 1983, and STS-41B, Feb. 3-11, 1984.

SPARTAN was a NASA/GSFC program, a retrievable free-flyer platform, which started in the early 1980s. This free-flyer platform (an autonomous sub-satellite, three-axis stabilized) provided short-term LEO observation opportunities for instrumentation in various disciplines and fields of applications, such as astronomy, remote sensing, and technology demonstrations. 44)

The SPARTAN platform is a small, rectangular, free-flying vehicle, measuring roughly 1 m x 1.25 m x 1.5 m. It is released from the Shuttle and picked up after several days of conducting its experiments. Several SPARTAN platform configurations have been built so far.

Mission duration

2-5 days, depending on mission requirements

Deployed mass

up to 1400 kg

Platform dimensions

Mission dependent (up to a length of 2.3 m) a rectangular pallet

Power

Self-contained subsystem, 28±5 VDC, 3 A max, 7 kWh available

Instruments

Mission-dependent

Attitude control

Three-axis stabilized with cold gas thrusters, max slew rate : 1º/s2

Thermal control

Active and passive, SPARTAN electronics limited to 0º - 50ºC

Data rates

Mission dependent

Data storage

Onboard recorder

Table 8: Some characteristics of the SPARTAN platform series 200

Shuttle Flight

SPARTAN Payload

Comments

STS-51G, June 17 - 24, 1985

2 X-ray proportional counters

SP-A (mass of 1100 kg)

STS-51L, Jan. 28, 1986 Challenger accident

SP-Halley, 203, (UV-spectrometer)

STS-56, April 8 -17, 1993, ATLAS-2

UVCS, WLC, solar physics

SP-201-1, (mass of 1360 kg)

STS-64, Sept. 9 - 20, 1994

UVCS, WLC, solar physics

SP-201-2, reflight of SP-201-1 timed to coincide with Ulysses south polar pass

The SPARTAN-201 series (five missions) is dedicated to NASA's solar physics program. Scientific data are collected during each mission using a tape recorder and, in many cases, film cameras. There is no command and control capability after deployment. Power during the deployed phase of the mission is provided by on-board batteries, and attitude control is accomplished with pneumatic gas jets. Retrieval of the SPARTAN platform at the end of each mission by the Shuttle.

• SPARTAN-201-1 (STS-56, April 8 -17, 1993, mass of 1360 kg). On Apr. 11, 1993, the crew used RMS (Remote Manipulator Arm) to deploy SPARTAN-201-1 (retrieval on Apr. 13). Objective: monitoring of the solar-wind acceleration by observing the hydrogen, proton and electron temperatures and densities, and the solar-wind velocities in a variety of coronal structures at locations from 1.5 to 3.5 solar radii from the sun. The main instruments of the SPARTAN-201-1 flight are UVCS and WLC.

UVCS is an instrument of SAO (Smithsonian Astrophysical Laboratory, Cambridge, MA). Measurement of line profiles/intensities of Lyman-alpha (1215 Ä) and intensities of the Oxygen VI lines (1031.9 and 1037.6 Ä). The measurements are used to determine velocities, temperatures and densities of the coronal plasma in the regions observed. 45)

WLC (White-light Coronagraph):

of GSFC, built by NCAR. Measurement of polarization/ intensity of white-light corona. The measurements allow determination of electron densities in the coronal features observed. The WLC instrument is the most recent version of the spaceborne externally occulted Lyot coronagraph. WLC uses a rotating half-wave plate, a linear polarizer, a serrated occulting disk for occulting the disk at 1.25 solar radius, and a linear, wide dynamic range detector CCD camera to measure the intensity and polarization of broad band visible coronal radiation. The distance between pixel centers of the CCD camera is 22.5 arcseconds. Observations out to 6 solar radii from sun-center were obtained within a sector of 60º in width centered on the UVCS radial scan capability.

22) “Calibration of Long Term Satellite Ozone Data Sets Using the Space Shuttle,” E. Hilsenrath, in Optical Remote Sensing of the Atmosphere, 1990 Technical Digest Series of the Optical Society of America, Vol. 4, pp. 409-412

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.